The objective of this project is to characterize mutational events associated with insertion and excision of the transposon P element at the RNA polymerase II locus, RpII215, of Drosophila. Several P element insertion mutations which affect the expression of this gene have been analyzed at the molecular level. The insertions are clustered in two small subintervals of the locus, one region within coding sequences and the other region near the 5' end of the gene. Detailed analyses have focused on two of these insertions, D50 and W42, both of which lie near the 5' end of RpII215 and reversions of D50 which restore variable amounts of activity to the gene. Wild type DNA in the 5' region of the RpII215 gene has been sequenced. Within this region the sites of insertion of mutants D50 and W42 have been localized by sequencing. Although both insertions lie within the same region, the precise site of W42 insertion is 80 base pairs upstream of the D50 insertion site. DNAs from four D50 revertants have been cloned and sequenced; these were shown to be the result of either partial or complete loss of P element. Of the partial reversions two mutants, whose deletion breakpoints differ only slightly from each other, retain a portion of the inverted repeat sequences from the ends of P element. Another revertant was generated by internal deletion but retains almost 400 base pairs of P element DNA. The level of activity in these revertants seems to be determined by the nature of P element sequences that remain rather than the amount of P element DNA.